1. Field of the Invention
The present invention relates to a reflective film formed by using a cholesteric mixture and a method of making such a reflective film. The present invention also relates to a display device including such a reflective film.
2. Description of the Related Art
A reflective film of the type utilizing the selective reflection characteristics of a helical structure that is formed by a cholesteric liquid crystal material has been known in the art. The wavelength λ of selective reflection of a reflective film like this is obtained by multiplying together the refractive index n of a cholesteric liquid crystal material and the helical pitch P of its helical structure. That is to say, λ≈n×P. This selective reflection is observed in either right-handed circularly polarized light or left-handed circularly polarized light. Accordingly, by using a stack of two reflective films for reflecting right-handed circularly polarized light and left-handed circularly polarized light, respectively, the reflected light can be utilized highly efficiently. Thus, by utilizing the reflection characteristics of the helical structure formed by a cholesteric liquid crystal material, a polarizer of a polarization separation type, a color separation film (i.e., polarizing color filter) and a reflective liquid crystal display device including such a polarizer or color separation film have been developed.
However, the wavelength range of incoming light that is selectively reflected by a cholesteric layer (which will be herein referred to as a “selective reflection wavelength range Δλ”) heavily depends on the refractive index anisotropy Δn of the material for the cholesteric layer. Accordingly, it has been difficult to arbitrarily control the selective reflection wavelength range Δλ or to realize a broad selective reflection wavelength range Δλ.
To overcome this problem, Broer et al. proposed a technique of forming a cholesteric layer having continuously changing helical pitches P in Nature, Vol. 378, 467 (1995). By taking advantage of this technique, Merck & Co., Inc. produces an optical element “Transmax” that realizes polarization separation over the entire wavelength range of visible radiation. The technique of forming such a cholesteric layer broadens the selective reflection wavelength range by utilizing the phenomenon that the helical pitch of a cholesteric layer changes continuously with the concentration distribution of a polymerizable compound as measured vertically to the surface of the layer (i.e., in the thickness direction of the layer). The concentration distribution of the polymerizable compound is formed by the diffusion of the molecules of the polymerizable compound while the cholesteric layer is being formed by the ploymerization of the polymerizable compound.
As used herein, the “cholesteric liquid crystal material” means any liquid crystal material that exhibits a cholesteric phase. Thus, the “cholesteric liquid crystal material” includes not only a cholesteric liquid crystal in a narrow sense but also a mixture of a nematic liquid crystal material and a chiral agent. A material that can be used to form a reflective film according to the present invention exhibits both a cholesteric phase and polymerizability. Typically, the material includes a polymerizable compound, a compound such as a polymerization initiator and so on in addition to the cholesteric liquid crystal material. Thus, such a material that includes the cholesteric liquid crystal material and exhibits polymerizability will be herein referred to as a “cholesteric mixture”. It should be noted that if the cholesteric liquid crystal material itself includes a polymerizable compound, then the cholesteric mixture may consist essentially of the cholesteric liquid crystal material alone. Also, a layer that has a helical structure formed by the cholesteric liquid crystal material and that is obtained by polymerizing the polymerizable compound of a cholesteric mixture will be herein referred to as a “cholesteric layer”.
However, the present inventors discovered and confirmed via experiments that even when a cholesteric layer was formed by using such a polymerizable compound, the cholesteric layer sometimes had an insufficiently high reflectance due to the creation of an excessive number of domains in the cholesteric layer. The present inventors also discovered that such a cholesteric layer sometimes failed to broaden the selective reflection wavelength range sufficiently because of a very small variation in the helical pitch of its helical structure.